Summary

The event Multi-Processing Module (MPM) is
designed to allow more requests to be served simultaneously by
passing off some processing work to supporting threads, freeing up
the main threads to work on new requests. It is based on the
worker MPM, which implements a hybrid
multi-process multi-threaded server. Run-time configuration
directives are identical to those provided by
worker.

To use the event MPM, add
--with-mpm=event to the configure
script's arguments when building the httpd.

Topics

See also

This MPM tries to fix the 'keep alive problem' in HTTP. After a client
completes the first request, the client can keep the connection
open, and send further requests using the same socket. This can
save significant overhead in creating TCP connections. However,
Apache HTTP Server traditionally keeps an entire child process/thread waiting
for data from the client, which brings its own disadvantages. To
solve this problem, this MPM uses a dedicated thread to handle both
the Listening sockets, all sockets that are in a Keep Alive state,
and sockets where the handler and protocol filters have done their work
and the only remaining thing to do is send the data to the client. The
status page of mod_status shows how many connections are
in the mentioned states.

The improved connection handling may not work for certain connection
filters that have declared themselves as incompatible with event. In these
cases, this MPM will fall back to the behaviour of the
worker MPM and reserve one worker thread per connection.
All modules shipped with the server are compatible with the event MPM.

A similar restriction is currently present for requests involving an
output filter that needs to read and/or modify the whole response body,
like for example mod_ssl, mod_deflate, or mod_include. If the
connection to the client blocks while the filter is processing the
data, and the amount of data produced by the filter is too big to be
buffered in memory, the thread used for the request is not freed while
httpd waits until the pending data is sent to the client.

The MPM assumes that the underlying apr_pollset
implementation is reasonably threadsafe. This enables the MPM to
avoid excessive high level locking, or having to wake up the listener
thread in order to send it a keep-alive socket. This is currently
only compatible with KQueue and EPoll.

This MPM depends on APR's atomic
compare-and-swap operations for thread synchronization. If you are
compiling for an x86 target and you don't need to support 386s, or
you are compiling for a SPARC and you don't need to run on
pre-UltraSPARC chips, add
--enable-nonportable-atomics=yes to the
configure script's arguments. This will cause
APR to implement atomic operations using efficient opcodes not
available in older CPUs.

This MPM does not perform well on older platforms which lack good
threading, but the requirement for EPoll or KQueue makes this
moot.

To use this MPM on FreeBSD, FreeBSD 5.3 or higher is recommended.
However, it is possible to run this MPM on FreeBSD 5.2.1, if you
use libkse (see man libmap.conf).

For NetBSD, at least version 2.0 is recommended.

For Linux, a 2.6 kernel is recommended. It is also necessary to
ensure that your version of glibc has been compiled
with support for EPoll.

The event MPM handles some connections in an asynchronous way, where
request worker threads are only allocated for short periods of time as
needed, and other connections with one request worker thread reserved per
connection. This can lead to situations where all workers are tied up and
no worker thread is available to handle new work on established async
connections.

To mitigate this problem, the event MPM does two things: Firstly, it
limits the number of connections accepted per process, depending on the
number of idle request workers. Secondly, if all workers are busy, it will
close connections in keep-alive state even if the keep-alive timeout has
not expired. This allows the respective clients to reconnect to a
different process which may still have worker threads available.

This directive can be used to fine-tune the per-process connection
limit. A process will only accept new connections if the current number of
connections (not counting connections in the "closing" state) is lower
than:

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.